The present invention relates to a position detecting device for detecting the position of an object and, more particularly, to an MR position detecting device for detecting the position of an object required to be detected with high resolution (accuracy), such as the position of a zoom lens or a focus lens disposed in the lens-barrel of the camera within a video camera by the use of a magnetoresistive (MR) element. The invention, further relates to a position detecting device for providing a position detecting signal with high resolution by interpolation of position detection signals output from MR elements for a plurality of phases.
An object whose position is required to be detected with high resolution, there is the position detection of a zoom lens or a focus lens within a video camera. The lens-barrel of a video camera is constructed, for example, of four groups of lens systems to achieve the zoom function and autofocus function. The lens systems of the first group and third group are fixedly disposed, while the lens system of the second group for zooming and the lens system of the fourth group for focusing are movably disposed within the lens-barrel. The zoom lens is driven, for example, by a stepping motor and the focus lens is driven, for example, by a voice motor.
In a video camera, in general, position control is executed such that the focus lens is shifted corresponding to the movement of the zoom lens and the position of image formation is kept fixed at all times. Especially in the "manual zoom tracking" mode in which the zoom lens is manually moved, the focus lens must be shifted such that its position traces a certain preset locus and the focus lens must be positioned with accuracy within about 20 .mu.m.
Conventionally, in detecting the position of the zoom lens and focus lens, a method has been used in which the number of steps of a stepping motor is calculated and the position is indirectly detected therefrom. Another method uses a potentiometer and the traveling lens position is detected from the change in its resistance.
When potentiometers are used, since inequalities of resistance value are produced in the potentiometers, complex adjustment work for compensating for the inequalities must be made for individual video cameras. A problem arises since a large quantity of labor is required when such adjustment work is to be done on a large number of video cameras. Further, such a problem is encountered since the resistance value of the potentiometer varies with changes in temperature and, hence, its accuracy is lowered even after the above described adjustments are made. Furthermore, there is a problem that power is consumed for driving the potentiometer and, hence, a loss is produced in the thrust for the lens system and reliability is lowered.
On the other hand, the method in which the number of steps of the stepping motor is calculated cannot be applied to a lens for which a stepping motor is not used, i.e., to the focus lens in the above case. Further, in such a method in which the number of steps of the stepping motor is counted and the position is thereby indirectly detected, sometimes a disadvantage exists in that the phase of the input current and the angle of rotation does not have a one-to-one correspondence due to such factors as the detent. Therefore, a position different from the actual position becomes detected. Especially in this position detection method, a problem exists in that once an error was made during a course of position detection, the detection error remains unless the detection is reset to the reference position.
So far discussion has been made about a video camera taken as an example, but such problems as mentioned above are also encountered in position detections with other instruments where high resolution is required.